The present invention is a layered electrophotographic photoconductor, i.e., a photoconductor having a metal ground plane member on which a charge generation layer and a charge transport layer are coated, in that order. Although these layers are generally separate from each other, they may be combined into a single layer which provides both charge generation and charge transport functions. Such a photoconductor may optionally include a barrier layer located between the metal ground plane member and the charge generation layer, and/or an adhesion-promoting layer located between the barrier layer (or ground plane member) and the charge generation layer and/or an overcoat layer on the top surface of the charge transport layer.
In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing an area of the surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and those unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners containing pigment components and thermoplastic components. The toners, which may be liquids or powders, are selectively attracted to the photoconductor surface, either exposed or unexposed to light, depending upon the relative electrostatic charges on the photoconductor surface, development electrode, and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to the photoconductor surface, pulling the toner from the photoconductor surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor surface. A set of fuser rollers melts and fixes the toner in the paper, subsequent to direct transfer or indirect transfer when an intermediate transfer medium is used, producing the printed image.
The electrostatic printing process, therefore, comprises an ongoing series of steps in which the photoconductor surface is charged and discharged as the printing takes place. It is important to keep the charge voltage and discharge voltage on the surface of the photoconductor relatively constant as different pages are printed to make sure that the quality of images produced is uniform (cycling stability). If the charge/discharge voltage is changed significantly each time the drum is cycled, i.e., if there is fatigue or other significant change in the photoconductor surface, the quality of the pages printed will not be uniform and will be unsatisfactory. Similarly, if the surface or other parts of the photoconductor undergo wear, particularly uneven wear, during the course of the printing process, the pages printed will not be uniform and the quality of the final product unsatisfactory.
It has now unexpectedly been found that addition to the charge transport layer of low surface energy polyolefin waxes having a mean particle diameter of from about 6-12.mu., reduces positive electrical fatigue and end seal and paper area wear in a photoconductor.
Organic and inorganic particles are known for use as charge transport dopants and for inclusion in various photoreceptor layers to improve wear. Particulates which have been disclosed for this use include low surface energy additives, such as polyolefins and fluorine-containing polymers (such as PTFE), as well as high surface energy additives, such as hydrophobic silica. Thus, U.S. Pat. No. 5,096,795, Yu, issued Mar. 17, 1992, teaches that the use of particulate materials in the charge transport layer of a photoconductor improves wear resistance and resistance to stress cracking while maintaining the good electrical properties of the photoconductor. Particles utilized include microcrystalline silica, polytetrafluoroethylene (PTFE), and micronized waxy polyethylene. Particles utilized in the charge transport layer have a diameter between about 0.1 and about 4.5.mu., with the average particle diameter being about 2.5.mu.. It is taught that the particles are actually screened to remove larger particles such that the particles used fall within the defined particle size ranges.
U.S. Pat. No. 5,485,250, Kashimura, et al., issued Jan. 16, 1996, describes an electrophotographic imaging member having a surface layer comprising a binder resin and fluorine- or silicon-containing particles. The particles utilized include tetrafluoroethylene and polydimethyl siloxanes and have a diameter of from about 0.01 to about 5.mu., preferably from about 0.01 to about 0.35.mu.. These devices are said to provide color images of improved quality.
U.S. Pat. No. 5,714,248, Lewis, issued Feb. 3, 1998, describes an electrophotographic imaging member that includes a coating comprising a resin, electrically conductive metal oxide particles and insulative particles (such as fumed silica, which is preferred, undoped zinc oxide and undoped titanium dioxide).
U.S. Pat. No. 5,733,698, Lehman, et al., issued Mar. 31, 1998, describes an electrophotographic photoreceptor, which is said to control beading of the toner carrier liquid on the photoreceptor surface, comprising an electroconductive substrate, a photoconductor layer, an interlayer, and an outer release layer. The surface of the release layer must have at least a minimum roughness that may be provided by incorporation of filler materials including polystyrene beads and acrylic particles (having a particle average diameter of from about 10 to about 50,000 nm).
U.S. Pat. No. 5,021,309, Yu, issued Jun. 4, 1991, describes the inclusion of particulate materials in the anti-curl layer of an electrophotographic imaging system to provide a reduced coefficient of surface friction and improved wear resistance without adverse effects on the optical or mechanical properties of the system. The particulate materials disclosed include fluorocarbon polymers, fatty amides, polyethylene waxes, polypropylene waxes and stearates, having a particle size diameter range of from about 0.1 to about 4.5.mu., with an average particle diameter of about 2.5.mu..
U.S. Pat. No. 5,686,214, Yu, issued Nov. 11, 1997, describes an electrophotographic imaging system that includes a ground-strip layer comprising a dispersion of conductive particles and solid organic particles in a film-forming binder. The organic particles disclosed include micronized waxy polyethylene particles having a particle size of from about 0.1 to about 5.mu..
U.S. Pat. No. 5,725,983, Yu, issued Mar. 10, 1998, describes the inclusion of a mixture of inorganic and organic particles in the charge transport layer, anti-curl layer or ground-strip layer of an electrophotographic photoreceptor. Useful organic particles disclosed include waxy polyethylene particles having a diameter in the range of from about 0.1 to about 4.5.mu., with an average particle diameter of about 2.5.mu..
U.S. Pat. No. 4,784,928, Karr, et al., issued Nov. 15, 1988, describes the inclusion of particles in the outer layer of an electrophotographic imaging element to enhance the release of toner from the element onto paper. Particles which are described as useful in this regard include tetrafluoroethylene and polyolefin waxes. There is no discussion of particle size, but they appear to be quite small; in one example, the particle size is 2.mu. and the entire layer formed is only 0.1.mu. thick.
U.S. Pat. No. 5,385,797, Nagahara, et al., issued Jan. 31, 1995, describes an electrophotographic imaging member that includes an outer protective layer comprising a binder resin and a particulate electroconductive material coated with a siloxane compound. The particles utilized in this layer are very small having a diameter of less than about 0.3.mu., preferably less than about 0.1.mu..
U.S. Pat. No. 5,504,558, Ikezue, issued Apr. 2, 1996, describes an electrophotographic imaging member which includes a fluorine-containing particulate resin in its surface layer. The particle sizes used are from about 0.01 to about 10.mu., preferably about 0.05 to about 2.mu.. There is no suggestion to include a particulate resin in the charge transport layer. The essence of the invention is in the selection of specific binder resins for the surface layer and the photosensitive layer so as to provide good image quality with greater durability.
U.S. Pat. No. 5,610,690, Yoshihara, et al., issued Mar. 11, 1997, describes an electrophotographic imaging member having a lubricative resin powder in its surface layer and a spacer member in contact with that surface layer. This structure is said to provide good image quality without damaging the surface layer or causing it to separate from the photosensitive layer. Particulates disclosed as being useful include fluorine-containing resin powders (which are preferred), polyolefin resin powders, and silicon-containing resin powders.
As can be seen, none of these patents disclose photoconductor elements which include relatively large polyolefin waxy particles having a particle size of from about 6-12.mu. in their charge transport layer. In fact, the prior art suggests that particles in excess of 4.5.mu. create problems in a photoconductor context by scattering incident light or by harming the photoconductor electrical properties.